1. Field
The following description relates to a conductive paste or ink containing silver nanoparticles, and a conductive circuit board having a circuit wire formed using the paste or ink.
2. Description of Related Art
Information communication devices, such as liquid crystal displays, are becoming more miniaturized with higher performance. Attempts have been steadily made to incorporate these devices on flexible material supports. The circuit wire of the devices is generally formed by forming a film by vapor deposition such as chemical vapor deposition (CVD), sputtering and so on, and etching out an unnecessary portion by photolithography and so on.
However, the conventional method for forming a circuit wire has disadvantages caused by repetition of film formation and etching, such as a low usage efficiency of raw materials, generation of a large amount of waste, a long manufacturing time, and a considerable installation cost. Further, the conventional method encounters many difficulties in forming a fine circuit wire required for miniaturization of the aforementioned devices.
To solve the above-referenced issues, recently, the related industries have been focusing on ink-jet printing or a roll printing. These printing techniques allow a low loss of raw materials, non-use of hazardous components such as lead or the like, and a simple process for forming a circuit wire. More than anything else, these techniques support formation of a thinner and finer circuit wire than the conventional techniques. However, to form a circuit wire by said techniques, development of a high-performance conductive paste or ink is needed.
A conductive ink suitable for forming a circuit wire should have a specific resistivity of 1×10−5 Ω·cm or less for a high conductivity. However, it is not preferable to use a costly conductive filler so as to achieve said specific resistivity.
Further, when printing the conductive paste on a flexible circuit board material such as polyethylene terephthalate (PET) and so on, a sufficiently low sintering temperature is important, because plastics or the like have a low glass transition temperature (Tg). The sintering temperature is set depending on the characteristics of a conductive filler (generally metal particles) and other components of the conductive paste. However, as the smaller metal particles have higher surface energy, the sintering temperature tends to be even less than an intrinsic melting point of a metal.
Silver has a high conductivity, and, thus, is suitable as a conductive filler of a conductive paste for forming a fine circuit wire. However, the use of a large amount of silver increases cost. Further, silver has a poor adhesion with a widely utilized circuit board material and a difficulty in forming a smooth circuit wire.
In addition, silver cannot achieve both a low sintering temperature and a low cost at the same time. It is preferred to use silver having a particle size of nanometer or smaller level so as to lower the sintering temperature of the conductive paste. However, in this case, it requires more cost than the use of silver having a particle size of micrometer or larger level. Conventionally, silver was used at an amount of 50 weight % or more or 80 weight % or more to the maximum based on the weight of the conductive paste so as to attain a desired level of conductivity of the conductive paste. If the content of silver does not come up to the range, a gap is created between silver particles, so that the silver particles are not electrically connected. Although there are electrical contacts between silver particles, the number of electrical contacts is very small, causing insufficient conductivity. In the case of screen printing, it is easy to control the viscosity of the conductive paste having a high content of silver to a suitable level for spray through a mesh net. However, a high content of silver nanoparticles having a high tendency to agglomerate increases cost, and needs an additive such as a dispersant, a stabilizer or the like, to ensure storage stability of the conductive paste. Because the additive generally has a high molecular weight of 10,000 or more, the use of the additive increases the sintering temperature again. That is, the sintering temperature reduction effect obtained by reducing the particle size of silver is minimalized due to the increased silver content for a high conductivity.
For these reasons, the conventional conductive paste containing silver uses a high concentration of silver to form a continuous electrical network, thereby lowering a specific resistivity down to 1×10−5 Ω·cm or less. And, when the particle size of silver is 20 nm or less (level of silver nanoparticles), the conventional conductive paste has a minimum limit of sintering temperature of about 150° C. In summary, the conventional conductive paste has the limited particle size of silver to achieve a low sintering temperature and a high electrical conductivity. And, a large amount of silver having a particle size of a predetermined level or smaller causes side effects of a cost rise and a high sintering temperature resulted from the use of an additive such as a stabilizer. Under these circumstances, there is still a demand for a conductive paste that solves the problems and has a good adhesion with a circuit board material and a suitable viscosity for printing.